gh-127787: refactor helpers for `PyUnicodeErrorObject` internal interface (GH-127789)

- Unify `get_unicode` and `get_string` in a single function.

- Allow to retrieve the underlying `object` attribute, its
  size, and the adjusted 'start' and 'end', all at once.
  Add a new `_PyUnicodeError_GetParams` internal function for this.
  (In `exceptions.c`, it's somewhat common to not need all the attributes,
  but the compiler has opportunity to inline the function and optimize
  unneeded work away. Outside that file, we'll usually need all or
  most of them at once.)

- Use a common implementation for the following functions:

  - `PyUnicode{Decode,Encode}Error_GetEncoding`
  - `PyUnicode{Decode,Encode,Translate}Error_GetObject`
  - `PyUnicode{Decode,Encode,Translate}Error_{Get,Set}Reason`
  - `PyUnicode{Decode,Encode,Translate}Error_{Get,Set}{Start,End}`